(1) Field of the Invention
The present invention relates to use of a primer to promote the bonding of non-polar or highly crystalline substrates with adhesives, especially with cyanoacrylate adhesives. In particular, the primer of the present invention promotes bonding of polyolefins or similar plastic substrates with low surface energy.
Because of their low surface energy non-polar materials are difficult to bond, even with cyanoacrylate adhesives. These materials are represented by linear polymers of simple olefins, such as polyethylene, polypropylene, polybutene and the like, including their copolymers. Other materials which can be bonded by this invention are substrates having low surface energy such as polyacetals, plasticized PVC (polyvinyl chloride), polyurethane, EPDM (ethylene-propylene diamine monomer) rubber, thermoplastic rubbers based on vulcanized polyolefins and the like.
The materials bonded by this invention have, in general, a waxy, paraffin-like surface character, with a low critical surface tension of wetting and may have at their surface a weak boundary layer. In general, there is an absence of polar groups which are necessary to raise the surface energy to the level needed to give adhesion with conventional adhesives.
(2) Description of the Prior Art
Various surface treatments have been used heretofore to improve adhesion of the difficult-to-bond materials described above. These treatments include surface abrasion, corona discharge (J. C. Von der Heide & H. L. Wilson, Modern Plastics 38 9 (1961) p 199), flame treatment (E. F. Buchel, British Plastics 37 (1964) p 142), U.V. radiation (Practising Polymer Surface Chemistry, D. Dwight, Chemtech (March 1982) p 166), plasma treatment, hot chlorinated solvents, dichromate/sulphuric acid and chlorine/U.V. radiation (D. Brewis & D. Briggs, Polymer 22 7 (1981) and references therein). All the above treatments are effective to some degree and are generally thought to work by introducing polar groups on non-polar substrates.
While the above treatments may, on occasion, improve the ability of difficult substrates to be bonded with adhesives, the treatments have the disadvantage of requiring specialized apparatus and equipment and can involve corrosive and/or hazardous materials.
Apart from the above treatments, there are examples in the prior art of the use of various accelerators or initiators of the polymerization of the cyanoacrylate monomer. The disadvantage with such accelerators or initiators is that while the speed of cure of the bond is increased, adhesion on non-polar substrates is not promoted and the resulting bond can be very weak and the adhesive can slip off the substrate.
An accelerator comprising a solution of certain amines and a deodorant is disclosed in Japanese patent application No. 599066471-A. U.S. Pat. No. 3 260 637 of Von Bramer discloses the use of various classes of organic amines (but specifically excludes primary amines) as accelerators in conjunction with cyanoacrylate adhesives. The compositions of both of these specifications can best be described as accelerators or initiators of the polymerization of cyanoacrylate monomer without improving adhesion on non-polar substrates.
Various primers also have been proposed to promote adhesion. For example, Japanese patent application Nos. 52-98062, 56-141328 and 57-119929 suggest the use of primers prepared by dissolving a chlorinated polyethylene or chlorinated polypropylene or a fatty acid-modified acrylated alkyd resin in an organic acid. In general, however, organic acids tend to inhibit or prevent curing of cyanoacrylate adhesives. The primer of the present invention has the advantage of using a low molecular weight material which is readily soluble in common solvents.
European specification No. 0 129 069 A2 discloses the use of a primer comprising at least one organometallic compound, for use with alphacyanoacrylate adhesives, which is useful in bonding non-polar substrates. Japanese application No. 61023630 proposes organometallic primers for joining plastic models. Both of these documents disclose the use of organo-metallic compounds which are potentially highly toxic materials.
Primers for polyolefins comprising (i) modified or grafted polyolefins such as chlorinated polyolefins, ethylene/vinyl copolymer or ethylene/propylene terpolymer, (ii) a crosslinkable binder and (iii) crosslinking agents, are disclosed in European application No. 187171A. The primer described there is somewhat inconvenient to use and apply due to its rubbery consistency, and heating may be needed to activate the primer on the substrate. In addition the solvents used (e.g. toluene and 1,1,1-trichloroethylene) have an undesirable toxicity.
German application No. DE 3601518-A suggests the use of primers for polyolefins comprising an organometallic compound and an organic polymer; the organometallic compound being a metal alcoholate and the term "metal" including metalloids, phosphorous, boron or silicon atoms. The materials disclosed such as dibutyltin diacetate and other organo-metallic compounds are highly toxic for use as primers.
Japanese application No. 61136567 describes a titanate primer for bonding polypropylene. The titanate primers give little or no improvement when bonding difficult materials, such as polyethylene, to itself or to other materials, when cyanoacrylate adhesives are employed.
Japanese application No. 60157940 discloses a primer composition based on a solution of 4-vinyl pyridine as an essential ingredient, and 3,4-lutidine as an optional ingredient, for bonding polyolefins, polyacetal, polyamide, polyester, boron-polymer, silicone or PVC. However, the disadvantage with the primer of Japanese application No. 60157940 is that it has a strong odour and, in addition, it does not produce as strong a bond as that produced by the primer of the present invention.